[RLlib] Issue 41370: Tune update_config should use config.to_dict() if config is an _Config.

[simonsays1980]

Why are these changes needed?

When storing the params.json for RLlib experiments the config is of class AlgorithmConfig and not dict. Because of this the params.json contained only an object description, but not the content. This PR fixes this by calling the to_dict()-method of the AlgorithmConfig.
Other config classes can also implement a to_dict method such that this solution is well generalizable.

Everything that is dict remains dict.

Related issue number

Closes #41370

Checks

• I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• I've run scripts/format.sh to lint the changes in this PR.
• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • Unit tests
  • Release tests
  • This PR is not tested :(

[sven1977]

LGTM.